The present invention relates generally to optical lithography elements and blanks, and particularly to optical microlithography crystal lens elements and optical element blanks for use in optical photolithography systems utilizing vacuum ultraviolet light (VUV) wavelengths below 200 nm, preferably below 193 nm, preferably below 175 nm, more preferably below 164 nm, such as VUV projection lithography systems utilizing wavelengths in the 157 nm region.
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 200 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 200 nm, such as 193 and 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in these VUV excimer laser regions through optical materials. Available optical materials and their optical properties at such short lithography wavelengths have hindered the industrial utilization of the below 200 nm wavelengths in optical lithography systems. The utilization of fluoride crystals such as calcium fluoride as optical lens blanks for and optical lens elements in optical lithography has been hindered by the birefringence of the fluoride crystal optical materials and the detrimental effect that the crystal""s birefringence has on the lithography light transmitted though it. For the benefit of vacuum ultraviolet photolithography in the below 200 nm wavelength 157 nm region such as the emission spectrum VUV window of a F2 excimer laser to be utilized by the semiconductor industry in the manufacturing of integrated circuits there is a need for optical lithography fluoride crystal lens element members and optical member blanks with minimal birefringence.
European Patent Application EP 1001314A2 of the Nikon Corporation describes a durable optical system for projection exposure which utilizes calcium fluoride crystals for their superior durability against high irradiating lithography energy densities such as produced by a ArF excimer lithography laser. U.S. Pat. No. 6,061,174 of the Nikon Corporation describes a lithography image focusing optical system for ultraviolet lasers with shorter than 300 nm wavelengths which utilizes image focusing calcium fluoride crystal optical members with low sodium concentrations. Such systems have utilized calcium fluoride crystal lens elements with uniform  less than 111 greater than  crystallographic orientation direction as shown in FIG. 4 herein. The prior art has not addressed the adaption of the fluoride crystal element to the lithography light beam it is manipulating.
The present invention overcomes problems in the prior art and provides beneficial lithography elements with beneficial optical properties and lithography characteristics including minimal birefringence and blanks therefor that can be used to improve the lithographic manufacturing of integrated circuits with VUV wavelengths.
An embodiment of the invention comprises a birefringence minimizing fluoride crystal VUV optical lithography lens. The lens is preferably comprised of a single fluoride crystal. The fluoride crystal lens has an optical center axis encompassed by a fluoride crystal lens perimeter with the lens having a variation in crystallographic orientation direction which tilts away from an optical center axis  less than 111 greater than  direction orientation and towards the perimeter.
In further embodiment the invention includes a fluoride crystal optical lens. The fluoride crystal optical lens is comprised of a single fluoride crystal having a first optical lens surface and a second optical lens surface which opposes the first lens surface. The fluoride crystal has a progressive concentration of crystal dislocation defects that increases from the first optical lens surface to the second optical lens surface.
A further embodiment of the invention includes a calcium fluoride crystal optical element for manipulating a transmitting wavelength xcex less than 200 nm. The calcium fluoride crystal optical element comprises a calcium fluoride crystal having a first optical surface and a second optical surface for manipulating the wavelength xcex light. The calcium fluoride crystal has a concentration of crystal dislocation defects that increases from the second optical surface to the first optical surface. The first optical surface has a first surface subgrain boundary length per unit area in the range from about 10 to 50 cm/cm2. The second optical surface has a second surface subgrain boundary length per unit area  less than 10 cm/cm2.
An embodiment of the invention includes a fluoride crystal lens blank comprising a fluoride crystal having a center axis through a large dimension D surface. The center axis and the large dimension D surface are encompassed by a blank perimeter. The center axis aligns and coincides with an optically preferred crystallographic direction of the fluoride crystal. The fluoride crystal lens blank has a variation in crystallographic orientation direction which spreads out from the center axis and tilts away from the center axis and towards the blank perimeter.
A further embodiment of the invention includes a birefringence minimizing fluoride crystal optical lithography lens blank. The blank comprises a calcium fluoride crystal having a 193 nm internal transmission  greater than 99%/cm, a 157 nm internal transmission  greater than 99%/cm and a refractive index homogeneity no greater than 1 ppm. The calcium fluoride crystal has a plurality of non-parallel crystallographic  less than 111 greater than  orientation and the blank has a center axis which coincides with a calcium fluoride crystal  less than 111 greater than . The blank has a crystal perimeter and a variation in crystallographic orientation direction wherein the angular deviation between the center axis and a given non-parallel crystallographic  less than 111 greater than  orientation having a relative location between said center axis and the perimeter correlates to the relative location distance from the center axis. Preferably the angular deviation increases with an increase in distance from the center axis.
Another embodiment of the invention includes a fluoride crystal optical element blank. The optical element blank is comprised of a fluoride crystal having a diameter, a first diameter plane surface, a second diameter plane surface, and a crystal thickness defined by the first and second surfaces. The blank lens has a concentration of crystal dislocation defects that increases from the second surface to the first surface. The second surface has a second surface subgrain boundary length per unit area SS and the first surface has a first surface subgrain boundary length per unit area F S wherein FS greater than SS.